S. Soled and A. Wold in "Crystal Growth and Characterization of In.sub.2/3 PS.sub.3 ", Mat. Res. Bull. Vol. 11, pg. 657-662, 1976, Pergamon Press, Inc., discuss in their introduction a number of mixed anion rich compounds of metal, pnictide and chalcogenides. They report the work of W. Klingen, Dissertation, Universitat Hohenheim, Germany, 1969, dealing with the crystal growth of the compounds M.sup.II PX.sub.3 (with M = Fe, Co, Ni, Zn, Mn, Cd, Sn, Hg or Pb; X = S or Se) by means of chemical vapor transport. They go on to indicate that these materials are structurally related to the layer compounds CdI.sub.2 and CdCl.sub.2 and contain close packed sulfur layers with every other interlayer filled with an ordered arrangement of metal atoms and sigma-bonded phosphorus-phosphorus pairs. The metal atoms are located in octahedral interstices and each phosphorus atoms is bonded in a distorted tetrahedron to three sulfur and one phosphorus atom. Because of the large anion-anion interlayers that remain empty (with a typical sulfur-sulfur interplanar distance of 3.4 A), these compounds exhibit easy cleavage parallel to the crystal faces and exhibit lubricity.
It was not recognized, however, that such material possesses and retains this lubrication capability under oxidizing conditions at relatively high temperatures and perform satisfactorily over periods of time which greatly exceed the operational times of conventional lubricants such as MoS.sub.2.
In SLE Transactions, 14, 62, (1970) by Jamison and Cosgrove, the lubricating characteristics of a number of layered transition metal disulfides and diselenides were measured. The coefficient of friction was determined using a ball on flat type test apparatus loaded to 250g force. The static wear member was a 3/8 inch steel sphere. The lubricants were hand burnished onto a brass disc which acted as the dynamic wear member.
For the layered transition metal compounds which were studied, three types of lubricating behavior were observed. Some materials did not form adherent films; others did form films but could not support a sliding load; yet others formed films which could support heavy sliding loads. Table I summarizes the results. This clearly demonstrates that not all layered materials and more particularly not all layered sulfides are effective as solid lubricants.
TABLE I ______________________________________ STRUCTURE AND LUBRICATING PROPERTIES OF A NUMBER OF LAYERED TRANSITION METAL SULFIDES AND SELENIDES Coefficient of Dynamic Composition Structure Friction (250 g Load) ______________________________________ ZrS.sub.2 1T No Film.sup.a ZrSe.sub.2 1T No Film.sup.a NbS.sub.2 3R No Film.sup.a NbS.sub.2 2H No Film.sup.a NbS.sub.2 3R No Film.sup.a NbSe.sub.2 2H 0.075 NbSe.sub.2 2H 0.058 MoS.sub.2 2H 0.045 MoS.sub.2 2H 0.040 MoSe.sub.2 2H 0.057 HfS.sub.2 1T No Film.sup.a HfSe.sub.2 1T No Film.sup.a TaS.sub.2 4H Failed.sup.b TaS.sub.2 3R No Film.sup.a TaSe.sub.2 2H Failed.sup.b WS.sub.2 2H 0.051 WS.sub.2 2H 0.053 WSe.sub.2 2H 0.047 WSe.sub.2 1T No Film.sup.a WSe.sub.2 2H 0.037 ReS.sub.2 3R Failed.sup.b ReSe.sub.2 Dist. 1T Failed.sup.b ______________________________________ .sup.a No lubricant film could be formed. .sup.b Lubricant film broke dow before reaching this load.